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Serum transferrins from two holocephalan, five elasmobranch and three teleost species have been compared using quantitative microcomplement fixation. Calculated immunological distances emphasize the close relationship between the holocephalans and elasmobranchs and strongly support the view that they should be considered as part of a natural assemblage which is widely separated from the teleosts.

If the Holocephali and elasmobranchs have been separate since the beginning of the Carboniferous, this implies that transferrin has evolved at a rate approximating to 0-15-0-26 immunological units per million years involving some 9–15% substitution of amino acids. These values are extremely low and indicate that holocephalan and elasmobranch transferrins have evolved some 2–3–7 times more slowly than those known from any other group of vertebrates.